First, let me explain the Church line on contraceptives. The Catholic Church believes that you can't do anything that will prevent conception from happening. This is why the Church frowns upon birth control, pulling out, etc., because it prevents conception and there are passages in the Bible condemning that. Popular birth control methods, such as the Pill, are also abortifascients (causing you to basically have an abortion, not prevent a pregnancy) and can lead to higher risks of breast cancer and miscarriages down the road.

The Catholic Church's approved way of 'birth control' is Natural Family Planning, or NFP. A lot of people confuse this with the Rhythm Method, but they're completely different. NFP was created by a doctor, and is almost foolproof. You can only get pregnant certain times of the month, and other times you can't get pregnant at all (it's about half and half, unlike the misconception that NFP means you can only have sex two or three days a month). So basically, you get an NFP trainer who works with you for the first few months until you get the hang of it, and then you can use NFP to either specifically try to get pregnant or avoid getting pregnant. If you stick to it and follow the rules, it works, and supposedly, couples who use NFP tend to have more sex long-term in their marriages than couples who don't. (When you go through Pre-Cana, the required pre-marital counseling in the Catholic Church, a mini-tutorial on NFP is part of it, which is how I learned about it).

Here's a website you can look at if you want to learn more:
http://www.bygpub.com/natural/natural-family-planning.htm

All that said -- not every Catholic is orthodox. Not every Catholic toes the line on what the Church says we should do. Different parishes and priests will be more conservative or liberal. And it isn't like we have to send in surveys every year saying we do or don't use birth control. There are Catholics who use birth control just like there are non-Catholics who do.